1. Field of the Invention
This invention relates to an optical disk.
2. Prior Art
There is a growing need for an optical disk having a higher density and a higher capacity. DVD (Digital Versatile Disk) is already commercially available, and the DVD has a storage capacity of about 4.7 GB per single side which is about seven times larger than the compact disk. Technologies enabling further increase in the amount of information recorded are under active development.
Technologies that have been used for increasing the recording capacity of an optical disk include use of a recording/reading beam having a reduced wavelength, use of an objective lens having a higher NA (numerical aperture) in the optical system irradiating the recording/reading beam, increase in the number of recording layers, and multi-value recording. Among these, three-dimensional recording by increasing the number of recording layers enables remarkable increase in the recording capacity at low cost compared to the use of shorter wavelength or use of a lens with a higher NA. The three dimensional recording medium is described, for example, in Japanese Patent Application Kokai (JP-A) 198709/1997, and JP-A 255374/1996 discloses a medium wherein a rewritable information storage layer and a read only information storage layer are laminated.
A multi-layer recording medium having a plurality of recording layer is usually provided with transparent resin layers between the recording layers in order to reduce crosstalk between adjacent recording layers. Attempts have been made to form such resin layer by spin coating. In the case of the multi-layer recording medium, the beam reaches the recording layer after passing through the resin layer, and returns to the optical pickup after being reflected on the surface of the recording layer. As a consequence, severe uniformity is required for the thickness, quality, and optical properties of the resin layer.
The resin layer formed by spin coating, however, suffer from the problem that the disk is thicker at its outer periphery compared to the radially inner region, namely, from the thickness inconsistency in the radial direction of the disk since surface tension only works toward the center of the disk at the outer periphery of the disk and this surface tension results in the excessive resin remaining at the outer periphery of the disk without dropping off the periphery. Increase in the number of the recording layers results in the increase in the number of the resin layers formed, and hence, in the accumulation of the thickness inconsistency. As a result, in the outer peripheral region of some of the recording layers, the reading beam reflected from the surface of the recording layer is not perpendicular to the disk substrate even if the reading beam had entered the medium at an angle perpendicular to the disk substrate. This results in the reduced amount of light returning to the optical pick up, and in some recording layers, the output in the reading will be significantly different between the region near the inner periphery and the region near the outer periphery.
In the case of an optical disk of bonded type wherein the recording layer is sandwiched between two substrates, the disk is prepared by depositing the recording layer and the resin protective layer on one of the substrate in this order, and the other substrate is bonded to the protective layer by using an adhesive or a sheet of pressure sensitive adhesive. When the protective layer is formed in such optical disk of bonded type by spin coating, the increased thickness at the outer periphery of the protective layer is likely to result in warping and distortion of the optical disk, and hence, in the difficulty of improving the mechanical precision of the disk. Although such poor mechanical precision is significant in the multi-layer recording medium, a single layer recording medium having only one recording layer also share the same problem.
The multi-layer recording medium also suffer from the problem as described below. In the case of the medium having a single recording layer formed on a substrate, the shape of the grooves (guide grooves) formed in the resin substrate will be transferred to the recording layer. In contrast, in the case of a medium wherein two or more recording layers are formed on the substrate with an intervening relatively thick transparent layer between the recording layers, it is quite difficult to transfer the shape of the grooves formed in the substrate to all of the recording layers since the groove depth is about 100 nm at most for optical reasons while the distance between the recording layers is far more than such groove depth. As a consequence, formation of the grooves in the transparent resin layer by photopolymerization (2P) process will be required as described, for example, in the JP-A 198709/1997 and eminent increase in the production cost is invited.
An object of the present invention is to provide an optical disk which has excellent mechanical precision. Another object of the invention is to provide a multi-layer optical disk having a plurality of data layers wherein every data layer exhibits sufficient properties in reading. Another object of the invention is to provide such multi-layer optical disk at a low cost.
Such objects are attained by the present invention as described in (1) and (2), below. (1) An optical information medium comprising a disk-shaped substrate or a pair of disk-shaped substrates, and at least one annular data layer for storing at least recorded information and at least one annular resin layer disposed on said disk-shaped substrate or between said pair of disk-shaped substrates, wherein
at least one of said substrate(s) has a configuration such that its surface facing toward the data layer is convex, and at least one of said resin layer(s) is formed such that it is thicker near its outer periphery compared to the region near its inner periphery.
(2) An optical information medium according to the above (1) wherein said medium has a data layer for storing the recorded information and a servo layer for storing servo information which is formed independently from said data layer.